In any manufacturing business, the need to keep inventory to a minimum, while retaining the capability of meeting customers' needs, is an important business objective. More specifically in the HVAC (heating, ventilating, and air conditioning) field, each manufacturer generally produces and stocks a large number of product models in order to provide the configurations and capacities needed to cool and heat structures such as residences and small commercial buildings. Other manufacturers concentrate on medium and large installations. It is common in the industry to provide sizes ranging from two through five ton cooling capacities in typical increments of 2, 2.5, 3, 3.5, 4, and 5 ton evaporator (cooling) or indoor coils with matching outdoor condensing units or outdoor heat pump sections.
To satisfy the variety of applications, the indoor blower coils or heating units that must be provided are in horizontal, vertical upflow, and vertical downflow (counterflow) configurations. To accommodate the size ranges for these three configurations for cooling alone, it would be necessary to stock as many as eighteen models. Additionally, where heating only is desired, individual models can be provided in a great number of sizes. In many cases, both heating and cooling are desired, and this can require either a heating unit complete with heat generating section and blower to which cooling coils are added or a cooling coil with a blower to which a heating section might be added. The product manual or literature of any major HVAC manufacturer will show an extensive listing of products.
In the past, combination air treating installations for heating, cooling, or both have generally been achieved by one of three basic design approaches. First, all-weather air treatment has been provided by adding a cooling coil unit downstream of the air flow from a furnace unit designed primarily for forced warm air heating. Second, it has been common to mount a cooling coil unit having its own independent blower in parallel flow relation to a furnace unit. In a third approach, combination air treating units have been constructed within a single unitary cabinet containing a furnace heat exchanger, a cooling coil unit, a blower unit, and appropriate control devices.
All of these approaches have necessitated substantial design compromises and have sacrificed the optimum efficiencies in either heating or cooling which might otherwise be obtained for each installation and its particular use demands. For example, a system that is designed primarily for forced air heating does not have sufficient air handling capacity to perform adequately for air cooling. The differences between heating and cooling requirements vary greatly in buildings of various types and different occupancies such that it is often difficult to obtain an optimum combination of heating and cooling by simply adding a cooling unit onto an existing forced air heating system.
Although the use of a separate blower-powered cooling coil added in parallel with an existing forced air furnace will provide both good heating and cooling performance, such structures have been expensive and bulky. In many homes, the available space for utility installations has not been sufficient for such parallel arrangements. Further, the use of flow selection dampers to change over between heating and cooling has introduced sources of air leakage, and such dampers are costly to install and to service.
The third approach of large and unitary combination structures has been unacceptable for general domestic use, because, from a manufacturing and sales standpoint, it is a practical impossibility to maintain an adequate inventory of the number of different combinations which would be required for various fuels and heating capacities, various cooling capacities, and various air handling rates. From the point of view of some customers, it has been an economic burden to initially purchase a complete combination unit rather than to first install a heating system and add cooling capability at some later date when the added expenses can be more easily assumed. Additionally, there are problems for installers in fitting such unitary combination units into existing utility spaces and for home builders to design the required space for such units.